In a hybrid communication system where a ground station communicates with both geostationary and low earth orbit satellites, the ground station antenna must be designed in order to facilitate communications with both types of satellites. Typically, the gain requirements of a ground station antenna which communicates with a geostationary satellite are different than the gain requirements of a ground station which communicates with a low earth orbit satellite. For communications with a geostationary satellite, it is generally required that antenna beams be high gain and narrow beam. High gain antenna beams are required due to the distance (36,000 Km) of the geostationary satellite from the ground station. Narrow beams are used in order to minimize the potential for interference with adjacent geostationary satellites. When communication with a second geostationary satellite is required, the antenna beam can be repositioned in order to establish communications with the second satellite.
For communications with a low earth orbit satellite, where the satellites are constantly in motion relative to the ground station, it is generally required that receive and transmit antenna beams be somewhat wider in beamwidth as well as requiring continuous beam scanning in order to maintain contact with the satellite. Since the low earth orbit satellites are closer to the ground station than geostationary satellites (between 500 and 1400 Km), receive and transmit antenna beams can be lower in gain. Further, it is generally desirable to use two antennas when communicating with a low earth orbit satellite system so that communications are not interrupted during hand-off from one satellite to another.
An electronically scanned phased array antenna has the potential to provide a small, aesthetically attractive, and reliable ground station antenna for communications with both geostationary and low earth orbit satellites. However, when a transmit or receive antenna beam is electronically scanned away from the natural boresight of the antenna, a degradation in antenna gain known as "scan loss" results. In addition to a degradation in antenna gain, the receive or transmit antenna beamwidth increases. These effects create difficulties in using the same antenna for communications with a low earth orbit satellite and a geostationary satellite when the same antenna is used. For communicating with a geostationary satellite, the loss of antenna gain makes the ground-to-satellite communications link difficult to establish and maintain. Additionally, any widening of an antenna beam can create an interference with neighboring geostationary satellites placed close to the desired satellite. Further, in a hybrid system with both geostationary and low earth orbit satellites as many as four antenna apertures could be required (1 antenna used for receiving signals from a geostationary satellite, 2 antennas for receiving signals from low earth orbit satellite, and 1 antenna for transmitting signals to a low earth orbit satellite).
Therefore, what is needed is a ground station antenna which can be used to provide receive and transmit beams which facilitate communications with both geostationary and low earth orbit satellites.